Improving the Performance of PL/SQL function calls from SQL Speaker: Tim Hall Event: OTN Tour 2016 - Guatemala

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Improving the Performance of PL/SQL Function
Calls from SQL
Tim Hall
Oracle ACE Director
Oracle ACE of the Year 2006
OakTable Network
OCP DBA (7, 8, 8i, 9i, 10g, 11g)
OCP Advanced PL/SQL Developer
Oracle Database: SQL Certified Expert
http://www.oracle-base.com
Books
Oracle PL/SQL Tuning
Oracle Job Scheduling

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What’s the problem?
 We sometimes need to call PL/SQL functions in the select list of
queries.
 By default, the function may be called for each row returned.
 If the function is called repeatedly with the same input parameters, this
can represent a massive waste of resources.
 Sometimes we are able to change the query, but not the
function. Sometimes we can change the function, but
not the query.
(setup.sql)

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Scalar Subquery Caching
 Rewriting function calls as scalar subqueries allows Oracle to
cache the results.
SELECT (SELECT slow_function(id) FROM dual)
FROM func_test;
 Oracle sets up an in-memory hash table to cache results of scalar
subqueries.
 The cache only last for the lifetime of the query.
 The cached values are not reusable in the current
session or in other sessions.
 (scalar.sql)

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DETERMINISTIC Hint
 The DETERMINISTIC hint has been available for a long time, but didn’t
seem to do much until 10g.
 Oracle will optimize calls to functions marked as DETERMINISTIC to
improve query performance.
 The caching is based on the array size of fetch, so mileage can vary.
 Cached return values only last for the lifetime of the call.
 The cached values are not reusable in the current session
or in other sessions.
 (deterministic.sql)

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11g Result Cache
 11g introduced two new caching features.
 Both share the same pool of memory in the SGA, controlled using:
 RESULT_CACHE_% parameters.
 DBMS_RESULT_CACHE package.
 V$RESULT_CACHE_% views.
(result_cache.sql)
 The Query Result Cache improves performance of complex queries that
return small number of rows. (query_result_cache.sql)
 The Cross-Session PL/SQL Function Result Cache improves
performance of function calls by caching the return values.
(plsql_result_cache.sql)
 Cached results can be reused in the same session and
in other sessions.

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Manual Caching Using PL/SQL Collections
 Caching of function calls is nothing new.
 Caching using PL/SQL collections has been done for many years.
 Collections are session-specific, but cached values can be reused in
multiple queries.
 (collection.sql)
 Manually caching can cause problems for volatile data.
 Remember, collections use memory. Don’t go nuts!

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Manual Caching Using Contexts
 Manual caching using contexts is similar to using collections.
 It shares many of the same drawbacks, but can allow a shared cache
between sessions.
 (context.sql)

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Scalar Subquery Caching (Revisited)
 Q: Do other caching methods make scalar subquery caching irrelevant?
 A: No. Alternative caching mechanisms don’t reduce context switching
between SQL and PL/SQL.
 (plsql_result_cache_2.sql)
 You should always use scalar subquery caching, even
when using other caching mechanisms.
 Scalar Subquery Caching reduces context switches,
but other methods have added benefits reuse
between queries and sessions.

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Is there anything relevant in 12c?
 Functions (and procedures) in the WITH clause. Reduces the context
switching associated with PL/SQL calls, which improves performance.
WITH
FUNCTION slow_function(p_id IN NUMBER) IS
BEGIN
RETURN p_id;
END slow_function;
SELECT slow_function(id)
FROM test_func;

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What about the FROM and WHERE clause?
 Function calls in the select list of an inline view follow the same rules as
for the main select list.
 If you are using table functions, consider switching to pipelined table
functions.
 Use Scalar Subquery Caching for functions in the WHERE clause if
possible.
 Avoid functions on indexed columns, or consider
function-based indexes.
(fbi.sql)

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What did we cover?
 Scalar Subquery Caching
 DETERMINISTIC Hint
 Cross-Session PL/SQL Function Result Cache
 Manual Caching Using PL/SQL Collections
 Manual Caching Using Contexts
 Function calls in the FROM and WHERE clause

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The End…
 Slides and Demos:
http://www.oracle-base.com/workshops
 Questions?